The present invention relates to heat staking machines for joining parts together, and more particularly to a device for use on such a machine and having a radiant heat source to heat and thereby soften the part to be deformed.
Heat staking is a process for permanently joining first and second parts at one or more discrete points marked by a plastic protrusion, hereinafter referred to as a stud, which extends upwardly from the first part and through an aperture in the second part when the second part is placed over the first part. The stud is sufficiently long to provide a volume of thermoplastic material which extends beyond the upper surface of the second part. Therefore, the plastic stud is heated until it is plastically deformable and then flattened and flared out with a metal punch to form a rivet-like head which locks the two parts together.
It is possible to accomplish the heating and the deforming of the stud simultaneously by heating the punch prior to bringing it into contact with the stud, the punch transferring its heat to the stud to soften it as it is being shaped. In such an operation, the punch is typically resistance heated by electrical current. Heat staking machines operating in this manner are disclosed in U.S. Pat. Nos. 4,767,298 and 5,227,173.
Another known heat staking technique is to heat the stud prior to it being contacted by the punch. In the past, this has been achieved by blowing hot air over the stud. U.S. Pat. No. 5,018,957 discloses a staking machine using electric heaters to generate the hot air and blowers to circulate the hot air over the stud. In some manufacturing operations, this pre-impact heating of the stud has been found to be advantageous in that it minimizes the amount of residual stress in the deformed stud after it has cooled. In the past, however, the apparatus necessary for the heating and circulation of air has resulted in a relatively large and mechanically complicated machine. Also, such a machine is relatively energy inefficient in that a large percentage of the heat generated is not transferred to the stud but rather is wasted. Moreover, the heat may be damaging to elements, such as printed circuits, on the parts being joined.
It is therefore desirable to provide a heat staking machine that is energy efficient and that is simple and compact in construction, and which overcomes the problems associated with prior devices.
The present invention addresses and solves the above-mentioned problems and meets the enumerated objects and advantages, as well as others not enumerated, by providing an apparatus for heat staking in which the stud is heated by precisely focused infrared energy. The apparatus comprises a housing for holding the apparatus and for defining a cavity which can be placed in such a position as to substantially surround the stud. An infrared energy source is affixed to the housing with an energy directing means for directing the energy to the stud for the purpose of softening the stud. A deforming tool, hereinafter referred to as a punch, is mounted on a moveable carriage and designed for movement relative to the energy source toward and away from the stud.
A preferred embodiment of the present invention hereinafter described utilizes at least one broadband incandescent lamp as the infrared energy source. This lamp is preferably a halogen lamp. The energy directing function is performed by one or more reflectors which are preferably gold plated to provide preferentially high reflectivity of infrared, thus increasing the percentage of total energy produced which reaches the stud.
In a specific embodiment comprising primary and secondary reflectors, the secondary reflector is segmented to direct infrared energy to different areas of the stud, thus distributing the infrared energy over a larger area of the stud and reducing the time required to produce the softened state.
In an alternate embodiment, the energy directing means includes a lens for focusing the energy into fiber optic cables. The fiber optic cables are arrayed around the stud, whereby the energy is directed from the cables onto the stud.